I. FIELD OF THE INVENTION
The present invention relates to radio communications. More particularly, the present invention relates to cellular radiotelephone communications in a code division multiple access system.
II. DESCRIPTION OF THE RELATED ART
Cellular radiotelephone systems enable radiotelephone subscribers to communicate with land-line telephone networks while moving through a geographical area. High density, high capacity cells in typical cellular radiotelephone systems are made up of three directional antennas centrally located in the cell. Each antenna typically radiates into a 120.degree. sector of the cell. A number of these cells are operated by a service provider to form a cellular radiotelephone system.
The cell shapes are determined by both the radiation pattern of the antennas and the local conditions at the cell site. Cells, however, are typically idealized as hexagonal patterns since such a pattern closely approximates the ideal antenna radiation pattern.
The Federal Communications Commission (FCC) governs the use of the radio frequency spectrum, deciding which industry gets certain frequencies. Since the RF spectrum is limited, only a small portion of the spectrum can be assigned to the cellular industry. This assigned spectrum, therefore, must be used as efficiently as possible in order to allow as many frequency users as possible to access the spectrum.
Multiple access modulation techniques are some of the most efficient techniques for utilizing the RF spectrum. Examples of such modulation techniques used in the cellular industry include: time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA).
CDMA modulation employs a spread spectrum technique for the transmission of information. A spread spectrum system uses a pseudorandom noise (PN) sequence that spreads the transmitted signal over a wide frequency band. This frequency band is typically substantially wider than the minimum bandwidth required to transmit the signal. The spread spectrum technique is accomplished by modulating each baseband data signal, that is to be transmitted, with a unique wide band spreading code. Using this technique, a signal having a bandwidth of only a few kilohertz can be spread over a bandwidth of more than a megahertz. Typical examples of spread spectrum techniques can be found in Spread Spectrum Communications, Volume I, M. K. Simon, Chap. 5, pp. 262-358.
In a CDMA-type radiotelephone system, multiple signals are transmitted simultaneously on the same frequency. A particular receiver then determines which signal is intended for that receiver by a unique spreading code in the signal. The signals at that frequency without the particular spreading code intended for that particular receiver appear to be noise to that receiver and are ignored.
Three different PN sequences, well known in the CDMA art, are used in CDMA radiotelephone systems: Walsh codes, long PN codes, and short PN codes. Each PN sequence has a different function in the system. The Walsh code is the spreading sequence used by the radiotelephone. The long PN code is a scrambling sequence used by the radiotelephone. The short PN code is used by both the radiotelephone and the base station as a co-channel identifier.
The short PN code is a 15 bit sequence that identifies each sector of each cell in the system. Since the same frequency and one PN sequence is used in the system, a different short PN offset differentiates each sector to preclude the radiotelephone from communicating with a sector that is too distant for quality communication. A more detailed explanation of the short PN offset and other aspects of CDMA communication are found in the CDMA interim specification from the Electronic Industries Association/Telecommunications Industry Association (EIA/TIA) IS-95.
The short PN offsets are typically reused throughout a CDMA cellular system in the same manner as frequencies are reused throughout an analog cellular system. The PN offsets are assigned to a cluster of cells. This cluster of cells is then reused multiple times within the cellular system. Each PN offset is not used by other nearby radiotelephones within a cluster as this would lead to interference on the channel and a reduction in signal quality. This interference is referred to in the art as co-PN offset interference.
Another type of interference experienced by cellular radiotelephone users is adjacent PN offset interference. This interference is due to the energy slipover between adjacent PN offsets.
Both types of interference affect the signal quality, referred to as the carrier to interference ratio (.sup.C /.sub.I). This ratio is the signal strength of the received desired carrier to the signal strength of the received interfering carriers. A number of physical factors can also affect .sup.C /.sub.I in cellular systems: buildings, geography, antenna radiation patterns, radiotelephone traffic transmitting power, and radiotelephone traffic location within the cell.
PN offset planning is a method for optimizing offset usage, optimizing reuse distance, and reducing interference. A PN offset plan also attempts to maintain adequate reuse distance to an extent where co and adjacent PN offset interferences are acceptable while maintaining an adequate .sup.C /.sub.I margin. A .sup.C /.sub.I .gtoreq.24 dB is required by the IS-95 specification.
In order to accomplish these diverse requirements, a compromise is generally made so that the target .sup.C /.sub.I performance is acquired without jeopardizing system performance. However, the existing PN offset planning does not permit this since the reuse pattern is based on cell cluster reuse in which the antenna directivity and PN offset planning are not coordinated. As a result, they exhibit poor .sup.C /.sub.I performance.
Moreover, the conventional cluster reuse scheme inevitably reproduces the same neighbor list in co-PN sites (the neighbor list is well known in the CDMA art and is described in greater detail in IS-95). This may cause the radiotelephone to mistake a distant cell site for a near cell site since both have the same PN offset. There is a resulting unforeseen need for an improved PN offset assignment scheme to enhance .sup.C /.sub.I by coordinating PN offset reuse and antenna directivity.